1. Technical Field of the Invention
The present invention relates generally to the field of optical modulation; and, more particularly, to a method and apparatus for velocity matching an optical signal propagating in a traveling-wave optical modulator with a modulating signal.
2. Description of Related Art
In optical communications systems, electro-optic and electro-absorptive optical modulators are often used to modulate an optical signal. One class of electrode used with electro-optic and electro-absorptive optical modulators is the “traveling-wave” type, wherein an RF electrode, such as a microwave electrode, for example, is positioned on top of or in parallel with an optical waveguiding modulator. The microwave signal represents data to be transmitted, and modulates an optical signal propagating in the waveguide by changing a property or properties of the optical signal. For example, the modulator may be configured to change the intensity of the light signal propagating in the waveguide in some relationship to the applied microwave signal.
To optimize coupling between the electrical field of the electrode and the optical field propagating in the waveguide, the microwave signal should propagate along the modulator with the same velocity as the optical signal. Microwave signals and optical signals, however, may travel through the modulator at different velocities, resulting in a reduction in efficiency and bandwidth. Typical InP-based optical modulators that are described in the literature, for example, report an optical velocity that is about twice the microwave velocity. The negative effects of such a velocity mismatch become increasingly significant as the length of the waveguide increases, as is the current trend in the industry.
There is, accordingly, a need for a method and apparatus for velocity matching an optical signal propagating in a traveling-wave optical modulator with a modulating signal.